The circuit diagram of FIG. 1 illustrates an electronic circuit 1 comprising a circuit 3 representing a crystal oscillator and a prior art drive circuit 5 for driving the crystal oscillator 3. As can be seen, the crystal oscillator is modelled by a first capacitor C1, a second capacitor C2, an oscillator inductor LO, a resistor RO, a third capacitor C3 and a fourth capacitor C4 connected in parallel with the second capacitor, the coil and the resistor. One of the electrodes of the first and third capacitors is connected to ground. In the circuit of FIG. 1, the drive circuit is modelled by an inverter circuit 7. This kind of drive circuit is currently widely used to drive crystal oscillators, for example. The inverter circuit 7 may comprise at least an n-type metal-oxide-semiconductor (MOS) transistor and a p-type MOS transistor, for example. The crystal oscillator and the drive circuit are connected so as to form a positive feedback circuit, which induces an oscillation.
The signal diagram of FIG. 2 illustrates the behaviour of the signals of the electronic circuit 1 of FIG. 1. It is to be noted that, to better illustrate the results, the resistor RO has been omitted. The first (top) graph illustrates the voltage of the second capacitor C2 as a function of time. In other words this graph illustrates the oscillation of the crystal oscillator. The second graph illustrates the output current of the drive circuit over time, while the third graph shows the drive circuit output voltage. The drive circuit 5 of FIG. 1 drives the crystal oscillator 3 for a full half-cycle of the oscillation. In the second graph in FIG. 2, it can be seen that the current from the drive circuit flows in both directions during a semi-period of oscillation, which means that the energy in the oscillator inductor LO of the crystal oscillator 3 is not increased and the voltage across the crystal oscillator does not increase. Assuming ideal conditions, i.e. no resistance and every component ideal, then the current provided by the drive circuit 5 must be dissipated in the drive circuit 5. In a real application this means that the energy is dissipated in the parasitic resistance of the transistors of the drive circuit 5.
Thus it becomes clear that the electronic circuit illustrated in FIG. 1 is not optimal in terms of power consumption. More specifically, the current flow from the driver 5 is not in phase with the current flow of the crystal oscillator 3, and this causes power dissipation in the transistors of the driver 5. A further problem may arise due to the brief but significant current which may flow in the inverter 7 during each switching transition of the inverter 7. Such a transition current spike may arise if the inverter comprises an NMOS-PMOS transistor pair as mentioned above, where this pair effectively presents a short-circuit when both are momentarily on.